I know Bard has been needing a new skin recently (as well as Ornn) and I thought I'd just give my personal ideas on this popular concept (edit: This sentence was misleading, I meant here's my take on that popular concept art which was made by somebody else).
https://pbs.twimg.com/media/ECbQjJxWwAI7svf.png:large (Fan art by: Glow, OCE server)
So, the pumpkin head is an obvious must.
Maybe the horn could be a witches hat, which Bard could wear in an emote.
Chimes can be goodie bags, or just various candy.
Meeps can be ghosts like in the art, or little zombies. I usually only draw on paper, but I decided to try and whip up a digital zombie here.
Its Q must be very bright. Could be a big ol' orange glowing mushroom, the cap being where he is and the stem extending from him. The stun animation can be just some generic bright stuff that matches the mushroom.
Its W can be a million things and we already know that. What I like is a candle raising into the bottom of a jack o' lantern. Could be some other things, maybe green alchemy flask filling with chemicals (which would be hard to make clear that it's Bard's heal). Also, he could place a tombstone+grave that raises out of the ground (one of the ones that are a cross+circle) and the cross glows green when complete.
Its E is the actually hard one, it must be bright but also have a big round symbol on 2 ends representative of halloween themes. I think the only real choice here is a jack o' lantern. Happy face on the entrance, evil on the other. I don't believe any other symbols would make more sense.
The ultimate could be a big cask he tosses in the air, purple poison lands on the ground with glowing zhonya's coloured veins running through it.
Now this would be a stretch, but if it was legendary it would be cool to have some non-Bard noises in his dialogue. This would be a lot of effort, but maybe swap his voice to the minor key (which matches the SR music) and make it a lot more shrill and if you use a waterphone it could save a lot of time.
It already sounds Bard-like honestly.
Just one last special request, play that first bit of Toccata and Fugue in D-minor for his taunt (everybody knows the song, just not the name). Regardless of key, if he doesn't do it then somebody else must.